Tectonic Synthesis, Leg 66: Transect and Vicinity

Abstract

Cores recovered from eight sites drilled along a transect across the Middle America Trench off southwestern Mexico show that accretion and offscraping of trench turbidites and the pelagic-hemipelagic blanket covering the ocean crust has been active for approximately 10 m.y. Active offscraping and imbricate underthrusting have been limited to the lower slope, whereas middle and upper slopes show uplift due to underplating by subducted sediments and younging of ocean crust. Offscraped trench sands of Pleistocene to Miocene age have been uplifted as much as 2 to 3 km above the present-day trench floor by combined accretion and underplating. Sediment input into the subduction zone is partitioned, with roughly equal amounts being offscraped, underplated, and subducted. The offscraped component appears to consist primarily of trench turbidites, whereas the subducted component is thought to consist primarily of pelagic-hemipelagic sediments. The trench turbidite component probably dominates the underplated component. Partitioning values based on mass balance budget and on theoretical lithospheric bending were comparable. This comparability lends confidence to our theory of flux partitioning. Although we find no direct evidence of tectonic erosion, there is indirect evidence to support the feasibility of the mechanism, and although there is no present forearc basin, differential uplift rates suggest incipient development. Thus we infer the following history of the region: 23-20 Ma—reorganization of plate motion: relative movement of Pacific and North American plate changes from oblique normal (subduction) to parallel (sinistral strike-slip) with margin subsidence and marine transgression. 19-17 Ma—margin sinks rapidly to, or just below, CCD, then begins slow rise. Sinistral strike-slip motion continues. ~ 10 m.y.—plate motion changes from strike-slip to normal-oblique; accretion begins and continues to present.